Thermal flow sensor having an electromagnetic actuator for a cyclic flow modulator

ABSTRACT

A thermal flow sensor is responsive only to a modulated component of the flow. At zero flow the modulated component disappears, except for an artifact caused by motion of the modulator. This artifact is minimized by reducing the extent of modulator motion and by sampling the modulated signal at a quiescent part of the modulator&#39;s operating cycle. This results in a thermal flow sensor with a very stable zero.

BACKGROUND OF THE INVENTION Background Information

Fluid flow rates are often measured using sensors that introduce flowimpedance. For example, differential pressure flowmeters, vortex meters,turbine meters, moving target meters and various other sorts of flowmeters configured as probes all restrict flow and thereby provide a flowimpedance that is an inherent feature of the sensor itself.

In a case of particular interest, a differential pressure sensor sensesa pressure difference caused by a flow obstruction, where thedifferential pressure is an indication of the fluid flow rate. Thedifferential pressure sensor's output is a highly non-linear function offlow rate and is usable over a limited operating range. Differentialpressure sensors are particularly inaccurate at very low flow rates.Thus, there is a need to enhance the linearity and accuracy of this typeof sensor at lower portions of the flow range.

A thermal flow sensor taught by the inventor in his U.S. Pat. No.6,023,969 is responsive to only a cyclic component of a modulated flow.Because the cyclic component, aside from a possible offset introduced byfluid motion induced by the flow modulator itself, is zero when there isno flow, this sensor provides an accurate and stable zero. This sensorshares a common drawback of thermal flow sensors in that the presence ofa contaminant film on a heated sensing element reduces accuracy athigher flow rates.

BRIEF SUMMARY OF THE INVENTION

One aspect of the invention is that it provides a composite apparatusfor measuring a rate of flow of a fluid in a primary conduit in theneighborhood of a flow impedance. This apparatus comprises a first flowsensor that may include the flow impedance as an implicit feature of itsinstallation in the primary conduit (e.g., the first flow meter may be avortex or turbine meter). Alternately, an orifice plate or other flowrestriction may be added to the primary conduit and the pressure dropacross that restriction may be measured by a flow sensor that does not,of itself, add an appreciable flow impedance and (e.g., a differentialpressure sensor) and that may require the addition of a separate element(e.g., an orifice plate) to provide the flow impedance. The first flowsensor is operable to provide a first measure of the rate of flow in theprimary conduit. The apparatus also comprises a bypass conduit extendingfrom a point upstream of the flow impedance to another point downstreamof the flow impedance so that a pressure drop in the primary conduitcauses fluid to flow through the bypass conduit. In addition there is athermal flow sensor installed in the bypass conduit. The thermal flowsensor comprises at least one heated temperature sensitive element and aflow modulator operable to modulate flow in the bypass conduit tothereby cause the at least one temperature sensitive element to generatea cyclic signal indicative of heat removed by the flowing fluid. Thecyclic signal is processed by signal processing circuitry that generatesfrom it a second measure of the rate of flow in the primary conduit.

Another aspect of the invention is that it combines a first flowmeasurement made by a flow sensor accurate at medium to high flow rateswith a thermal flow measurement having a stable zero. This can providean increased range of flows over which an accurate measurement can bemade. In this combination a preferred thermal flow sensor is generallyof the type described by the inventor in his U.S. Pat. No. 6,023,969,the teaching of which is herein incorporated by reference.

A specific aspect of the invention is that it provides an apparatus formeasuring the rate of flow of a fluid in a primary conduit by combiningat least: a flow restriction; a differential pressure sensor; and athermal flow sensor disposed in a bypass conduit extending between thetwo sensing surfaces of the differential pressure sensor. In thisarrangement the differential pressure sensor provides a first measure ofthe rate of flow of fluid in the primary conduit from the pressurechange associated with the restriction. The thermal flow sensor, whichis characterized by very low zero drifts, comprises a flow modulatoroperable to modulate flow in the bypass conduit so as to cause at leastone heated temperature sensitive element in the thermal sensor togenerate a cyclic signal indicative of the amount of heat removed fromit by the fluid flowing in the bypass conduit. This measurement can beextrapolated to provide a second measure of the rate of flow of fluid inthe primary conduit.

In a particular preferred embodiment, the invention provides anapparatus for measuring a rate of flow of a fluid in a primary conduit.This apparatus comprises: a flow restriction in the primary conduit; adifferential pressure sensor arranged to sense a pressure differentialcaused by the flow restriction and to provide therefrom a first measureof the rate of flow in the primary conduit; a bypass conduit extendingbetween the two sensing surfaces of the differential pressure sensor;and a thermal flow sensor for providing a second measure of the rate offlow. This thermal flow sensor comprises two heated temperaturesensitive elements disposed in portions of the bypass conduit andexposed to a modulated flow which causes the two temperature sensitiveelements to generate respective cyclic signals that are one hundredeighty degrees out of phase and which indicate the respective amounts ofheat removed by the flowing fluid. The thermal flow sensor alsocomprises thermal flow signal processing circuitry operable to receivethe cyclic signals and to generate from them a second measure of therate of flow. In addition, the preferred apparatus comprises decisionmaking circuitry having inputs from both the differential pressuresensor and from the signal processing circuitry. The decision makingcircuitry may be operable to construct a composite measurement of therate of flow from the first and second measures thereof, or to selectthat one of the two measures that is known to be the more accurateindicator of the measured flow rate. Moreover one of the meters can beused to calibrate the other.

It is yet a further aspect of the invention to combine a first flowmeasurement made in a primary conduit with a thermal flow measurementmade in a bypass conduit that has an orifice, valve or other restrictionin it. The flow restriction is preferably chosen to be small enough thatlittle fluid flows through the bypass channel and the readings from thefirst flow sensor are nearly unaffected by the secondary flow. Therestriction cuts the fluid flow increasingly as the flow rate increasesand responds to increasing flow rates in a manner complementary to theresponse of the first flow sensor so that the two measurements can becombined to provide a response that is both more linear and that extendsover a greater operating range. Note that the flow restriction in thebypass conduit can be located at a wide variety of locations within thatconduit.

A still further object of the invention is to provide a thermal flowsensor comprising thermal signal processing circuitry and a moreefficient and economical cyclic flow modulator. The cyclic flowmodulator may comprise a bifurcated flow channel; an electromechanicalactuator; a movable perforate member; and two heated temperaturesensitive elements. The perforate member is preferably movable by theelectromechanical actuator to provide a maximum flow rate to a firstportion of the bifurcated flow channel at the same time that it providesa minimum flow rate to the second portion of the bifurcated flowchannel. The two heated temperature sensitive elements may be separatelydisposed to receive fluid flowing through the first and secondbifurcated portions of the flow channel so as to generate respectivecyclic signals indicative of respective amounts of heat removedtherefrom by a flowing fluid.

Those skilled in the art will recognize that the foregoing broad summarydescription is not intended to list all of the features and advantagesof the invention. Both the underlying ideas and the specific embodimentsdisclosed in the following Detailed Description may serve as a basis foralternate arrangements for carrying out the purposes of the presentinvention and such equivalent constructions are within the spirit andscope of the invention in its broadest form. Moreover, differentembodiments of the invention may provide various combinations of therecited features and advantages of the invention, and that less than allof the recited features and advantages may be provided by someembodiments.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1A is a schematic depiction of a preferred flow measurementapparatus of the invention wherein a first flow sensor, which causes aflow restriction in a primary flow conduit, provides a first measure offlow and wherein a thermal flow sensor disposed in a bypass conduitprovides a second measure of flow.

FIG. 1B is a schematic depiction of preferred flow measurement apparatusof the invention wherein a differential pressure sensor, connectedacross a separate flow restriction in a primary flow conduit, provides afirst measure of flow and a thermal flow sensor installed in abifurcated bypass conduit provides a second measure of the flow.

FIG. 2 is a schematic block diagram depicting electronic signalprocessing and control functions associated with the apparatus of FIGS.1A and 1B.

FIG. 3A is a partly schematic, partly sectional view of a preferred flowmodulator from which some of the seals and supporting structure havebeen removed in the interest of clarity of presentation. FIG. 3B is aschematic top view of a portion of the preferred modulator of FIG. 3A,the view taken as indicated by the double-headed arrow 3B-3B in FIG. 3A.

FIGS. 4A and 4B are, respectively, detail views of a perforate rotor andstator pair used in the flow modulator of FIGS. 3A and 3B.

FIG. 5 is a cross-sectional view, taken as shown by the double-headedarrow 5-5 in FIG. 3A.

FIG. 6 is a schematic diagram of a circuit used to minimize the zerooffset of a thermal flow sensor of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In studying this Detailed Description, the reader may be aided by notingdefinitions of certain words and phrases used throughout this patentdocument. Wherever those definitions are provided, those of ordinaryskill in the art should understand that in many, if not most, instancessuch definitions apply both to preceding and following uses of suchdefined words and phrases.

Turning now to FIG. 1A one finds a schematic depiction of exemplarmeasurement apparatus of the invention 10 for measuring flow through apipe or other primary conduit 12. A flow impedance introduced by andinherent in the installation of a first flow sensor 14 provides apressure drop adequate to cause a small fraction of the flowing fluid toflow through a bypass conduit 16 connected to the primary conduit 12 atselected locations respectively upstream and downstream of the firstflow sensor 14. In addition, there is a thermal flow sensor 18 disposedin the bypass conduit 16.

In FIG. 1B, one finds a schematic depiction of an exemplar measurementapparatus of the invention for measuring flow through a pipe or otherprimary conduit 12. This apparatus comprises both a differentialpressure sensor 20 connected upstream and downstream of a separate flowrestriction 22 (which may be an orifice plate) by sensing tubes, and athermal flow sensor 18 disposed in a bypass conduit 16 extending fromone of the differential pressure transducer's sensing surfaces to theother.

As will become clear from the ensuing disclosure, the variousarrangements of valves 24 depicted in FIGS. 1A and 1B provide foroperation of both flow sensors and for the isolation of the thermal flowsensor 18 from the primary flow conduit when the thermal flow sensor isto be calibrated. The skilled reader will recognize that more or fewervalves, or valves located at other positions, may be used for anequivalent result.

In preferred embodiments, the bypass conduit 16 contains a small orifice26 or other restriction which may optionally be made adjustable with avalve, such as a needle valve. Fluid flow through the thermal flowsensor 18 is inhibited by the relatively small size of the orifice 26 sothat the rate of increase of flow in the bypass conduit 16 diminisheswith increasing pressure differential across the restriction 22 in theprimary conduit 12. This arrangement compensates for the increasingpressure differential of the fluid through the pipe as the flow rate inthe primary conduit increases.

Although FIG. 1B schematically depicts an orifice plate 22 restrictingflow in the primary conduit, the reader will appreciate that many otherstructures, which include, without limit, an adjustable post or wedge,may be used. Moreover, one could use a venturi structure as the flowrestriction, in which case one of the two sensing tubes could beconnected upstream of the venturi and the other at the position wherethe conduit diameter was a minimum.

The arrangements depicted in FIGS. 1A and 1B allow for outputs to beindependently generated by a first sensor 14 (which may be an in-linesensor or a differential pressure sensor) and by a thermal flow sensor18 that preferably uses two heated sensing elements 28 a, 28 b exposedto a modulated flow. These two outputs may be supplied to a controller30, which may be a microcontroller operating according to instructionsstored in a memory 32. The skilled reader will appreciate that manyother arrangements may be provided for comparing and evaluating theoutputs of the two sensors and that the controller is not constrained tobe in the immediate proximity of the two sensors.

The fluid flow rate through the thermal flow sensor 18 is generallyrestricted by the orifice 26 to be low and to hence have only a smalleffect on the flow measurement made by a differential pressure sensor20. However, in a preferred configuration a valve 24 is provided at aconvenient place in the bypass conduit 16 so that flow through thebypass conduit can be completely shut off. This enables the first sensorto respond to the flow rate without any loss of accuracy. Alternately,if both the sensors are operated simultaneously, a stored difference inthe readings of the first sensor with the valve opened and with thevalve closed can be used as a correction factor.

The thermal flow sensor 18 may be used by itself to detect fluid flowrate. It may also be operated from the same pressure sensing ports of anoperational differential pressure flow meter 20. Both meters can thenprovide an indication of the fluid flow rate at one location in thepipe. In addition, the thermal flow meter can use the output from adifferential pressure, turbine or vortex sensor for calibration purposesin flow regimes in which the differential pressure produced therefrom isan accurate indicator of flow rate. Note that when mass flow in theprimary conduit is to be measured rather than volume flow, thecalibration also requires inputs from temperature and pressure sensors(not shown).

When both sensors are used simultaneously, an overall flow rateindication can be generated by using an output from the first 14 (e.g.,differential pressure) sensor in a mid to upper flow rate range wherethat instrument is fairly accurate. The addition of the thermal flowsensor 18 allows for the apparatus to respond to lower flow rates and tohave a zero flow rate calibration point. The useful flow rate range ofthe apparatus can then extend from very low flow rates to the maximumrate of the differential pressure sensor. The turndown is therebyincreased from that characteristic of most sensors (e.g., about 10:1 fora differential pressure sensor) to several hundred to one, or more.

For the composite flow sensor of the invention 10 to have the desiredimprovement over other sensors, the thermal flow rate sensor 18 musthave very low zero drift. In a preferred embodiment, a thermal mass flowrate sensor is used which senses the fluid flow rate as it is variedfrom a maximum to a minimum by a modulator 34, as taught in theinventor's U.S. Pat. No. 6,023,969. Although the modulator 34 cancomprise a valve which is at one time opened to direct flow over asingle heated element and at another time closed, a preferred embodimentalternately diverts a single input flow stream between two electricallyheated elements 28 a, 28 b. This arrangement improves the speed ofresponse of the sensor and does not shut off the flow through themodulator.

As taught in the inventor's U.S. Pat. No. 6,023,969, mass flow ratesensing elements and supporting electronics that are responsive only tothe modulation component of the fluid flow are used for the ratemeasurement. At zero flow rate the modulation components of the fluidflow are also zero so that the sensing element output signal is zero,except for a small constant offset due to fluid movement caused by themodulator 34 itself. Because the thermal sensor only responds to themodulation components, its zero drift is resistant to the sensingsurfaces being affected by environmental contaminants, and by electricalcomponent drifts. The modulator 34 is therefore an important part in anexemplar metering system and this disclosure describes a modulator thatis improved with respect to the one disclosed in U.S. Pat. No.6,023,969.

It should be noted that the modulator 34 described herein is applicableto thermal flow sensors whether or not they operate with obstructions inthe primary line producing differential pressures. For example, themodulator could be used with a thermal flow sensor having an inlet tubefacing upstream and an outlet tube facing downstream to provide thefluid flow past the sensor. Furthermore, there is no inherent limit onthe size of the inlet and outlet tubes which, in some cases, can be asmall diameter bypass channel and in others can be the entire primaryflow channel

In some applications, notably for measuring steam flow, the sensingtubes and bypass conduit may experience ambient temperatures which willcause condensation that may interfere with the flow measurement. Forthese applications the tubes and bypass conduits may be intimatelylocated with the pipe or may be separately electrically heated so thatthey are maintained at least at the temperature of the fluid in theprimary conduit.

In some situations the fluid flowing in the primary conduit 12 maycontain substances that can affect the operation of the thermal flowsensor 18 either by partially or completely obstructing the orifice 26in the bypass conduit or by changing the characteristics of the sensor(e.g. by build-up of a contaminant film on an electrically heatedsensing element). To deal with these situations one may provide anoptional calibration loop 36 that may comprise a positive displacementcalibrating pump 38 and that may be connected to the thermal flow sensorwhen it is isolated from the primary conduit by suitable valves 24.

FIGS. 3A, 3B and 5 depict a preferred embodiment of an improvedmodulator 34. In this device the flowing fluid enters a single inputport 40, preferably centrally located, and is directed to either of thetwo output ports 42 a, 42 b that are preferably symmetrically disposedon opposite sides of the bypass conduit's flow axis. Each of the outputports 42 a, 42 b is respectively associated with a separate electricallyheated temperature sensitive element 28 a, 28 b. The fluid outputdirection is preferably controlled by the angular position of aperforated rotor body 44 coaxially disposed within a perforated stator46. The stator 46 and rotor 44 have slots 48 having the same axial andangular spacings so that the slots can be selectively aligned to allowflow or turned to block flow. These slots are angularly offset from eachother by the amount of the rotor rotation, as shown in FIG. 5. Hence thefluid flow alternates between the ports corresponding to the twopositions of he rotor. This provides that the flow past each of theelectrically heated temperature sensitive elements 28 a, 28 b ismodulated from a maximum value to a minimum, where the minimum need notbe a complete shut-off.

Although FIGS. 3A, 3B and 5 show a modulator in which the two outputports are in a facing relationship, the modulator can equally well bemade using output ports stacked along the axis of the input flow. Inthis case the rotor and stator both have two rows of slots, one abovethe other and offset one half of the angular pitch between slots, asdepicted in FIGS. 4A and 4B.

The modulator taught in U.S. Pat. No. 6,023,969 used an electric motorto rotate a rotor having two ports spaced apart by 180 degrees of arc.In the presently preferred embodiment only a relatively small angularchange of a mechanical component is required to alternate the fluid flowbetween the ports. Hence, the movement of the fluid due to themodulation process is also relatively low and any compensationadjustment required for correcting the flow indication, for example atzero flow rate, is also correspondingly small.

If the slots 48 are located around its entire circumference, the rotor44 may be rotated in one direction at a relatively low rate. The rotormay also be oscillated over the range of the angular distance betweenthe slots, in which case the slots need only be located in the vicinityof the exit ports.

In a preferred embodiment an oscillatory drive is provided by anelectromechanical actuator 50 such as that depicted in FIGS. 3A and 3B.A preferred actuator 50 device uses two pairs of permanent magnetsarranged with opposed polarities attached adjacent respective ends of anarmature 54. The use of dual pole pieces provides balance to themagnetic structure and the air gaps are minimized for improvedefficiency. It may also be noted that this arrangement provides lowpower consumption in that widely spaced drive pulses to the coil 56 canmove the armature 54 from one extreme position to the other and thearmature can be held in a selected extreme position by a respectivepermanent magnet 52 a, 52 b.

It may be noted that various electromagnetic actuators may provide thedesired oscillatory motion. For example, a magnetic drive using a singlemagnet can additionally use the pole pieces as mechanical stops. Thesepermanent magnet oscillatory drives are relatively simple and costeffective. However, other types of valve actuators which also actdirectly on the rotor such as stepper motors, torque and conventionalmotors may also be used.

The modulator may also use optical (photoelectric) or magnetic (halleffect sensor) means sensitive to the position of the rotor or any partattached to it (armature) with an electronic controller to control therotor movement so that it is precisely positioned at each end of itstravel.

Regardless of how the modulator is driven, the motion of the modulatingelement can generate a parasitic flow signal that appears as a zerooffset. This may be minimized by oscillating, rather than rotating, therotor and by detecting the output signal from each heated element 28 a,28 b just prior to the armature 54 repositioning. At these times theeffects of the fluid movement by the armature 54 are minimal. Preferredthermal signal processing circuitry 55 uses sample and hold circuits 58a, 58 b which provide signals to a differential amplifier 60, band passfilter 62 etc., as depicted in FIG. 6. Note that the modulationfrequency is on the order of a few Hertz, which is compatible with theoperating speed of the selected sample and hold circuitry.

In the preferred embodiment the flow modulator 34 is anelectromagnetically actuated mechanical valve which diverts the fluidflow alternately between two heated sensors 28 a, 28 b which provideoutput signals having magnitudes responsive to the magnitude of thefluid flow. A modulation controller 64 supplies the electrical power tothe actuator 50 and also through a delay 66, to a pulse generator 68which activates the sample and hold circuits 58 a, 58 b. The delay timeis set so that the sample and hold circuits 58 a, 58 b are activatedduring an oscillation dwell interval just before the flow is divertedfrom one sensor to the other. The outputs from the heated sensors arethen retained and enter a differential amplifier 60 to extract thedifference in the magnitude of their signals. These signals are bandpass filtered to remove any DC drift components and higher frequencynoise components. The filtered signal is supplied to a magnitude(amplitude) detector 70, the output of which is supplied to a low passfilter 72 providing the input to the output amplifier 74 which providesthe flow responsive signal from the thermal signal processing circuitry55.

Although the present invention has been described with respect toseveral preferred embodiments, many modifications and alterations can bemade without departing from the invention. Accordingly, it is intendedthat all such modifications and alterations be considered as beingwithin the spirit and scope of the invention as defined in the attachedclaims.

The invention claimed is:
 1. A thermal flow sensor for measuring flow ofa fluid, the sensor comprising: a cyclic flow modulator comprising: acommon flow channel portion and a bifurcated flow channel portion; anelectromechanical actuator; a perforate rotor turnable about an axis bythe electromechanical actuator; and a perforate stator co-axial with therotor, wherein the perforations in the rotor and stator cooperate toprovide a minimum flow impedance between the common flow channel portionand one of the two bifurcated flow channel portions when providing amaximum flow impedance between the common flow channel portion and theother of the bifurcated flow channel portions, each minimum flowimpedance state characterized by a plurality of respective perforationsin the rotor being aligned with a corresponding plurality of respectiveperforations in the stator; two electrically heated temperaturesensitive elements respectively disposed to receive fluid flowingthrough respective bifurcated channel portions, whereby the twotemperature sensitive elements generate respective cyclic signalsindicative of respective amounts of heat removed therefrom by theflowing fluid; and thermal flow signal processing circuitry operable toreceive the cyclic signals and to determine therefrom the rate of flow.2. The thermal flow sensor of claim 1 wherein the electromechanicalactuator is operable to turn the rotor through no more than 45 degreesof arc.
 3. The thermal flow sensor of claim 1 wherein the thermal flowsignal processing circuitry is operable to receive the respective cyclicsignal from each heated sensor at a time selected so that the effects offluid movement are minimal.
 4. The thermal flow sensor of claim 1wherein the thermal flow signal processing circuitry comprises sampleand hold circuitry.